Organic electroluminescent device and manufacturing method therefor

ABSTRACT

An anode, a light-emitting layer, and a cathode, which has a structure formed by sequentially laminating from the light-emitting layer side a first cathode composed of a material having a work function of 3.0 eV or less and a second cathode composed of a material having a work function higher than that of the first cathode so that the total thickness of the first and the second cathodes is 100 angstroms or less, are stacked on a substrate  1,  and light is emitted to the outside via at least the cathode.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to organic electroluminescentdevices which are electrical device light emitting devices used fordisplays, light sources for displays, and the like, and to manufacturingmethods therefor.

[0003] 2. Related Art

[0004] Recently, as self-luminous displays in place of liquid crystaldisplays, development of light-emitting devices (organicelectroluminescent devices, hereinafter referred to as organic ELdevices), which have a structure in which a-light-emitting layercomposed of an organic material in provided between an anode and acathode, has been advancing rapidly. Among these, a high-transmission ELdevice, a so-called transparent EL device (TOELD) used in the visiblelight region, which can emit light from the two electrode sides, hasbeen desired since overlapped displays can be performed by disposinganother display device thereunder, and a particular structure inproposed in, for example, Appl. Phys. Lett. 68(19), May 6, 1996, p 2606.In the paper mentioned above, it is disclosed that an aluminum complexAlq3, which is a low molecular material, is used as a light-emittinglayer, a cathode is formed by co-deposition of Mg and Ag, an ITO film isformed by sputtering thereon for sealing or for assistance to thecathode so as to form a device, and a threshold voltage of approximately8 V is achieved. In the structure described above, in view of the lifeand the threshold characteristics of a material used for thelight-emitting layer, Mg and Ag are used for the cathode, and inaddition, ITO is used for the upper layer thereon.

[0005] Concerning organic EL devices, it is proposed that alight-emitting material having a low threshold value is used for alight-emitting layer, and a metal material having a low work function isused for a cathode so as to realize operation at a low thresholdvoltage. However, in the structure proposed in the paper describedabove, Mg and Ag are not sufficient in view of the work functions, andsince ITO is additionally deposited on the metal material by sputteringin order to prevent the degradation thereof, Mg is oxidized, whereby aproblem may arise in that an increase in threshold voltage of the devicecannot be finally avoided.

SUMMARY OF THE INVENTION

[0006] The present invention was made in consideration of the problemdescribed above, and an object of the present invention is to provide anorganic electroluminescent device which can be operated at a low voltageand which has high efficiency, high transmission characteristics, and along life, and is to provide a manufacturing method therefor.

[0007] According to the present invention, an organic EL device isprovided, which comprises an anode , a light-emitting layer composed ofan organic material, and a cathode which has a structure in which afirst cathode composed of a material having a work function of 3.0 eV orless and a second cathode composed of a material having a work functionhigher than that of the first cathode are sequentially stacked from thelight-emitting layer side and the total thickness of the first and thesecond cathodes being 100 angstroms or less are stacked on a substrateand light is emitted to the outside via at least the cathode.

[0008] In addition, according to the present invention, a method formanufacturing an organic electroluminescent device is provided, whichcomprises the steps of; forming an anode on a substrate; forming alight-omitting layer composed of an organic material above the anode;and forming a cathode above the light-emitting layer by laminating afirst cathode composed of a material having a work function of 3.0 eV orless and a second cathode composed of a material having a work functionhigher than that of the first cathode from the light-emitting layer sideso that the total thickness of the first and the second cathodes is 100angstroms or less.

BRIEF DESCRIPTION OF DRAWINGS

[0009]FIG. 1 is a cross-sectional view showing an device structure of anorganic EL device according to an embodiment of the present invention.

[0010]FIG. 2 is a view showing a transmission spectrum in the visiblelight region of an organic EL device formed in Example 1 of the presentinvention.

[0011]FIG. 3 is a view of the -transmission spectrum showing timedependence of a change in transmittance of a device processed by anoxygen plasma treatment in an example of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0012] An organic EL device of the present invention is characterized inthat on an anode and a light-emitting layer composed of an organicmaterial provided on a substrate, a cathode is provided which has astructure in which a first cathode composed of a material having a workfunction of 3.0 eV or less and a second cathode composed of a materialhaving a work function higher than that of the first cathode aresequentially stacked with each other from the light-emitting layer side,in which the total thickness of the first and the second cathodes is 100angstroms or less, wherein light is emitted to the outside via at leastthe cathode.

[0013] As preferable nodes of the organic EL-device, the following arementioned.

[0014] (1) In the organic EL device, the cathode aide in scaled by asealing layer composed of a transmissive material.

[0015] (2) In the organic EL device, the first cathode comprises Ca.

[0016] (3) In the organic EL device, the thickness y (angstrom) of thefirst cathode is such that 50≦y≦80 holds.

[0017] (4) In the organic EL device, the thickness y (angstrom) of thefirst cathode is such that 55≦y≦65 holds.

[0018] (5) In the organic EL device, the second cathode comprises Al.

[0019] (6) In the organic EL device, the thickness a (angstrom) of thesecond cathode is such that 10≦z≦20 holds.

[0020] (7) Zn the organic EL device, ah organic material forming thelight-emitting layer is a polymeric material.

[0021] In addition, a method for manufacturing an organic EL device ofthe present invention comprises the steps of forming an anode on asubstrate; forming a light-emitting layer composed of an organicmaterial above the anode; and forming a cathode above the light-emittinglayer by laminating a first cathode composed of a material having a workfunction of 3.0 eV or less and a second cathode composed of a materialhaving a work function higher than that of the first cathode from thelight-emitting layer side so that the total thickness of the first andthe second cathodes is 100 angstroms or less.

[0022] As preferable modes of the method for manufacturing the organicEL device, the following are mentioned.

[0023] (8) In the step of forming the anode in the method formanufacturing the organic EL device, after an electrode film is formed,an oxygen or an air plasma treatment is performed under conditions inwhich a current x and a time t are set such that 10 (mA)≦x≦15 (mA) and 5(minute)≦t≦7 (minute) hold.

[0024] (9) In the step of forming the anode in the method formanufacturing the organic EL device, after an electrode film is formed,an oxygen or an air plasma treatment is performed under conditions inwhich a current x and A time t are set such that 10 (mA)≦x≦12 (mA) andt=5 (minutes) hold.

[0025] Hereinafter, the embodiments of the present invention will bedescribed in detail with reference to drawings.

[0026]FIG. 1 is a cross-sectional view showing the structure of anorganic EL device according to the prevent invention.

[0027] In the structure shown in FIG. 1, on a substrate 1, an anode 2, ahole injection/transport layer 3, a light-emitting layer 4 composed ofan organic material, a first cathode layer 5 composed of a materialhaving a work function of 3.0 eV or less, and a second cathode 6 (acathode is formed of a stacked structure of the first and the secondcathodes mentioned above) composed of a material having a work functionhigher than that of the first cathode are stacked. Next, the stackedstructure described above is sealed by a first sealing layer 7, and asecond insulating layer 8, and in addition, is sealed by a sealingsubstrate 9.

[0028] As the substrate 1, a transparent material, such as glass, or areflective material may be used. When the transparent material is used,light can be emitted to the outside via at least the substrate 1.

[0029] As a material used for the anode, for example, ITO or IDIXO(manufactured by Idemitsu Kosan Co., Ltd.) may be mentioned as atransparent electrode material. The material mentioned above isdeposited on a substrate so as to form an electrode by sputtering or thelike. The transparent electrode thus formed is preferably processed byan oxygen plasma treatment or an air plasma treatment after the materialis deposited.

[0030] The plasma treatment mentioned above is preferably performed at acurrent of 10 mA to 15 mA for 5 minutes to 7 minutes as a treatmenttime, and is more preferably performed at a current of 10 mA to 12 mAfor 5 minutes. When the treatment time is less than 5 minutes, inparticular, the transmittance of the wavelength in the range of 400 to550 nm is decreased by up to 3 to 5% compared to that in the case inwhich the treatment time is more than 5 minutes. In contrast, from 5minutes to 10 minutes for the treatment, an increase in transmittance inthe region of the wavelength-mentioned above is not observed. Forexample, when an oxygen plasma treatment is performed, it is preferablethat VPS020 manufactured by Sanyu Electron Co., Ltd. be used and thatthe treatment be performed after purging is performed 2 to 3 times byusing oxygen.

[0031] In addition, when the current is less than 10 mA the uniformityof surface treatment may be degraded in come cases, and when the currentis more than 15 mA, a decrease in film thickness may occur by ashing insome cases. In addition, a decrease in transmittance of the wavelengthin the range of 600 to 800 nm is observed by the treatment describedabove in accordance with the treatment time. Compared to the case inwhich the treatment is not performed, when the treatment is performedfor 10 minutes, a decrease in transmittance by up to approximately 2% isobserved. The reason for this is believed that a change in bandstructure is caused by oxidation of the surface of the transparentelectrode and by the generation of defects therein due to the treatmentdescribed above. Accordingly, an oxygen plasma treatment or an airplasma treatment is preferably performed approximately at 10 mA for 5minutes, and as a result, an increase in transmittance in the vicinityof 500 nm, to which humans are more sensitive, and an increase intransmittance on average in the visible light region can be realized.

[0032] In this connection, when the transparent electrode materialdescribed above is used for the anode 2, light can be emitted to theoutside via at least the substrate 1.

[0033] In addition, as the anode 2, for example, a layer composed of ametal, such as Pt, Ir, Ni, Pd, or Au, or a stacked structure of atransparent material layer composed of ITO or the like and a reflectivelayer composed of Al or the like may be used.

[0034] Furthermore, the substrate provided with the anode 2 formedthereon is preferably an active matrix substrate having a plurality ofanodes disposed thereon and switching devices, such as thin-filmtransistors, each provided for each anode.

[0035] In this embodiment, between an electrode 2 and the light-emittinglayer 4, the hole injection/transport layer 3 is provided. In thisconnection, the hole injection/transport layer is a layer having afunction of hole injection or hole transfer from the anode to thelight-emitting layer side. For the hole injection/transport layer 3described above, a mixture of polyethylenedioxy-thiophene

[0036] and polystyrene sulfonate,

[0037] copper phthalocyanine, or the like in preferably used.

[0038] For the light-emitting layer 4, a low molecular weight organiclight-emitting material or a polymeric light-emitting material may beused, a fluorene-based polymer is preferably used, and in particular,

[0039] a polymeric organic material, such as PPV(poly-p-phenylenevinylene) is preferably used.

[0040] Between the cathodes, for the first cathode 5, as describedabove, a material having a work function of 3.0 eV or less is used. Asthe cathode material mentioned above, in particular, Ca is preferablyused. Specifically, Ca is preferable since a low threshold voltagebecause of the low work function and a high transmittance because of thelow reflectance for visible light can be realized. The thickness of thefirst cathode described above is preferably from 50 to 80 angstroms, andmore preferably, from 55 to 65 angstroms. When the thickness is lessthan 50 angstroms, due to the influence of the work function of thesecond cathode 6, which is an upper layer, there may be a risk in thatthe threshold voltage of the device is increased. In addition, when itis 80 angstroms or more, there may be a risk in that the transmittanceis significantly decreased. In particular, in the case in which Ca isused, since Ca has absorption over almost the entire visible wavelengthregion, when the thickness thereof is excessively large, black tonebecomes significant throughout the cathode side. In particular, when theCa film has a thickness of approximately 80 angstroms, it is believedthat a continuous film is formed having a certain level of thicknesssuch that electrical conductance can be achieved. From this point ofview, it is also preferable that the thickness of the first cathode be80 angstroms or less. In addition, as the cathode, Au may also be used.

[0041] The first cathode described above can be formed by vacuumdeposition at a degree of vacuum of, for example, 1×10⁻⁶ torr or more.

[0042] In addition, between the cathodes, for the second cathode 6, amaterial having a work function higher than that of the first cathode 5is used. As a material therefor, a material, which has a work functionnot significantly higher than that of a material for the first cathode,has a certain level of stability to oxygen, and can easily form acontinuous film, is preferably used. There nay be mentioned, inparticular, Al and Ag.

[0043] In particular, when Ca in used for the first cathode, it ispreferable that Al or the like be used as a material for the secondcathode. The thickness of the second cathode described above inpreferably set to be 1 to 20 angstroms, and more preferably, is set tobe 10 angstroms. When it is less than 10 angstroms, electricalconductance cannot be obtained, and in addition, when it is more than 20angstroms, there may be a risk in that the transmittance issignificantly decreased by metal reflection of the material(particularly, a metal material) itself for the second cathode.

[0044] In the present invention, as described above, by forming a layercomposed of a material having a low work function (3.0 eV or less) asthe first cathode and by forming a layer, which is a continuous layerand has a work function higher than that described above, on the firstcathode as the second cathode, the degradation of the first cathode isprevented, and in addition, since the total thickness of the firstcathode and the second cathode, which form the stacked structure, isformed so as to be 100 angstroms or less, the light transmissioncharacteristics at the cathode side is ensured.

[0045] As a method for forming the stacked structure of the cathodesdescribed above, it is preferable that after the first cathode is formedby deposition, it be confirmed that the degree of vacuum reaches a levelapproximately equivalent to that at which the first cathode isdeposited, and the second cathode be then formed under conditionssimilar to those for the first cathode.

[0046] As the first sealing layer 7, for example, LiF, SiO, or SiO₂ isused. In particular, since a film of LiF can be easily formed by avacuum deposition method, the device can be formed without being exposedin the air from the film formation of the cathode to a subseqent seriesof operations in an inert atmosphere, and in addition, since thematerial itself contains no oxygen atom, conditions containing oxygen ata nearly zero concentration thereof can be maintained. Furthermore, thetransmittance in the visible light region is also high, and thetransmission characteristics are not degraded. The thickness and thedeposition rate are set to be 300 to 500 angstroms and 8 angstroms/secor more, respectively. When the thickness is less than 300 angstroms, itis difficult to protect the cathode, which is a lower layer, againstwater and oxygen from the outside air and the infiltration of water andair from the second sealing layer 8, which is an upper layer, bysealing. In addition, when the thickness of the film is more than 500angstroms, the device (in many cases, light-emitting layer) is damagedby heat radiation during deposition, and hence, there may be a risk inthat the intrinsic EL light-emitting characteristics are damaged.Furthermore, when the deposition rate is 8 angstroms/sec, since thedeposition time is long, device degradation also occurs by heatradiation an is the case described above. As a result, the depositionrate is also limited, and hence, it is required to have 8 angstroms/secor more.

[0047] As the second sealing layer 8, for example, a transparent heatcurable epoxy resin or a photocurable epoxy resin is used. Inparticular, a heat curable epoxy resin is preferable, and moreparticularly, coating thereof in performed by dipping, a glasssubstrate, which is the sealing substrate 9, is placed thereon, andcuring in then performed in an inert atmosphere, thereby forming thesealing layer. As the epoxy resin, a moisture-proof resin, such asDPpure60 (manufactured by 3M), or STYCAST1269A (Emeron), may bementioned.

[0048] Hereinafter, the present invention will be described in detailwith reference to examples.

EXAMPLE 1

[0049] An organic EL device having a structure shown in FIG. 1 wasformed.

[0050] Operations described below were all performed in a clean room.

[0051] On a washed glass substrate 1 of 150 mm square, an (transparent)electrode (anode) 2 (IDIXO) having 1,000 angstroms thick was formed bysputtering. The conditions therefor were; a degree of vacuum of 1×10⁻⁴Pa or less, an Ar to 02 flow ratio of 10:1, 320 V, 0.15 mA, and 14minutes. Next, an oxygen plasma treatment was performed on the anodefilm formed on the glass substrate at a current of 10 mA for 5 minutes.In particular, VPS020 manufactured by Sanyu Electron Co., Ltd. was used,and the treatment was performed after purging were performed 2 to 3times using oxygen.

[0052] Subsequent operations described below were performed in a glovebox. The conditions in the glove box were such that the oxygenconcentration was 0.01 ppm or less, and a dew point of water was −70° C.or less.

[0053] First, a mixture of PEDOT (polyethylenedioxy-thiophene) and PSS(polystyrene sulfonate) was applied as a hole injection/transportmaterial on the electrode 2 processed by the plasma treatment describedabove. The mixture described above could be obtained from Bayer A. G. asBaytron P. In this example, a solution was formed by mixing Baytron Pwith PSS in a ratio of 5:1 and by diluting the mixture with water to 1.5times, and by using the solution thus obtained, a film was formed byspin coating. Under conditions, such as a slope of 1 second, 3,000revolutions, and 45 seconds, a film 600 angstroms thick was formed. Byfiring the film at 200° C. for 10 minutes, film formation was performed,thereby yielding a hole injection/transport layer 3.

[0054] Next, on the hole injection/transport layer 3, a solution of afluorene-based polymer having a structure shown below dissolved in axylene solvent was applied by spin coating so an to have a filmthickness of 800 angstroms, thereby forming a light-emitting layer 4.

[0055] Subsequently, on the light-emitting layer 4, first, a filmformation (deposition) of Ca was performed in a vacuum depositionapparatus so as to form a first cathode 5. The vacuum depositionapparatus disposed !n the glove box was used. The degree of vacuum atthe beginning was approximately 1×10⁻⁶ torr. The deposition rate was setto be 3 angstroms/sec, and the film thickness was set to be 70angstroms. Next, after the degree of vacuum again reaches 1×10⁻⁶ torr, afilm 10 having angstroms thick was formed as the second cathode 6 bydepositing Al at a deposition rate of 3 angstroms/sec.

[0056] Next, on the second cathode 6, as a first sealing layer 7, a filmhaving 500 angstroms thick was formed by depositing LiF at a depositionrate of 8 angstroms/sec.

[0057] After cooling, on the first sealing layer 7, DPpure60(manufactured by 3M) was applied (film thickness, 200 μm) which was amoisture-proof epoxy resin, thereby forming a second sealing layer 8. Asealing glass (thickness, 0.3 mm) was then adhered thereto an a sealingsubstrate 9, and compressing was performed by a hot plate underconditions, at 50° C. for 12 hours, whereby curing was performed. Inthis step, in order to remove air bubbles in the epoxy resin, heatingwas performed at a degree of vacuum of approximately 0.1 torr. As aresult, an organic EL device was obtained.

[0058] For the organic EL device thus obtained, the transmittance andthe threshold voltage were measured. The measurement of thetransmittance was performed by using a spectroscope (manufactured byHitachi, Ltd.), in which air was used as the base line, and a pinhole of3 mm in diameter was provided at a condenser portion. The measurement ofthe threshold voltage was performed using BM-7 (manufactured byKabushiki Kaisha Topcon), and the threshold voltage was defined by avoltage at which a luminance of 5 Cd/m² is output.

[0059] The results are shown in FIG. 2.

[0060] According to the results shown in the figure, it was understoodthat the transmittance was 50% or more over almost the entire visiblelight region. In consideration of the glass substrate 1 having athickness of 1.1 mm and the transmittance thereof being approximately75%, it wan believed that a transmittance of 70% or more was achieved.In the case described above, the threshold voltage was 3 V.

EXAMPLE 2

[0061] In accordance with the method described in Example 1, glasssubstrates each having an anode stacked thereon were obtained, whereinthe anodes forced of transparent electrodes were processed by an oxygenplasma treatment for from 0 to 10 minutes at one-minute intervals. Forno glass substrates having anodes thereon, which were processed for 0, 5(Example), and 10 minutes, transmission spectrums (transmittance at eachwavelength) were measured. The results are shown in FIG. 3. In addition,for the glass substrates having anodes thereon, which were processed forfrom 0 to 10 minutes at one-minute intervals, transmittances at 450 nm,550 nm, and 700 nm were measured. The results are shown in Table 1.TABLE 1 Time for- an Oxygen Plasma Treatment 1 2 3 4 5 6 7 8 9 10 0(min) (min) (min) (min) (min) (min) (min) (min) (min) (min) (min) Trans-75.2 75.8 76.3 77 77.5 78.2 78.3 78.2 78.1 78.2 78.1 mittance (9; at 450nm) Trans- 74.6 74.7 74.9 75 75.2 75.2 75.3 75 74.8 74.5 74.3 mittance(9, at 550 nm) Trans- 75.8 75.7 75.5 75.5 75.3 75.2 75 74.9 74.6 74.474.3 mittance (9; at 700 nm)

[0062] In the blue region (450 nm), an increase in transmittance wasobserved until 5 minutes elapse from the start of the treatment. In thegreen region (550 nm), even though a significant change was notobserved, the transmittance exhibited the maximum value approximately 5minutes after the start of the treatment. In the red region (700 nm), asthe treatment time elapsed, the transmittance was decreased even thoughit was not significant. It is believed that the phenomenon describedabove occurred by a change of the band structure since the material waspartly changed due to the oxidation of the surface thereof by thetreatment. Accordingly, it was considered that a treatment time ofapproximately 5 minutes was an optimum condition.

EXAMPLE 3

[0063] Organic EL devices were formed in a manner similar to that inExample 1 except that the first cathodes were formed so as to havethicknesses of 40, 50, 60, 80, 90, and 100 angstroms. The thresholdvoltages and the transmittances at a wavelength of 550 nm were measuredfor the individual organic EL devices. In Table 2, the results of thethreshold voltages and the transmittances (a wavelength region of 550nm) are shown together with the results of Example 1 (the first cathodehaving a film thickness of 70 angstroms). TABLE 2 Film Thickness ofFirst 50 60 70 80 90 100 Cathode 40 (Å) (Å) (Å) (Å) (Å) (Å) (Å)Threshold 5 3.5 3.5 3 2.9 2.9 2.9 voltage (V) Transmittance 61 59 56 5450 45 38 (%, at 550 nm)

[0064] Concerning the threshold voltage, it was asymptomaticallystabilized at a film thickness of the first cathode of approximately 70angstroms. An increase in threshold voltage was observed when the filmthickness was not tore than that. It is believed that the phenomenondescribed above occurred by an increase in resistance caused by theinfluence of the work function of the second cathode or by theinsufficient film thickness of the second cathode.

[0065] The transmittance was decreased with the change in film thicknessof the first cathode, the acceptable level was up to approximately 90angstroms (a total thickness of the first and the second cathodes of 100angstroms), and at a level of the thickness of the first cathode higherthan that mentioned above (a level higher than that of a total thicknessof the first and the second cathodes of 100 angstroms), thetransmittance was excessively decreased. According to the resultsdescribed above, it was believed that a film thickness of approximately70 angstross of the first cathode was an optimum value.

EXAMPLE 4

[0066] Organic EL devices were formed in a manner similar to that inExample 1 except that the second cathodes were formed so as to havethicknesses of 5, 15, 20, 25, and 40 angstroms. The threshold voltagesand the transmittances at a wavelength of 550 nm were measured for theindividual organic EL devices. In Table 3, the results of the thresholdvoltages and the transmittances are shown together with the results ofExample 1 (the second cathode having a film thickness of 10 angstroms).TABLE 3 Film Thickness of Second Cathode 10 15 20 25 40 5 (Å) (Å) (Å)(Å) (Å) (Å) Threshold No Light Emission 3 3.1 3 3 3 voltage (V)Transmittance 57 54 49 43 32 20 (%, at 550 nm)

[0067] Concerning the threshold voltage, when the thickness thereof was10 angstroms or more, the change could not be observed when thethickness is 10 angstroms or less, it is believed that light emissionwas not performed since conductance could not be obtained. Since aconstant value was obtained, it was believed that there was no influenceof the work function of the second cathode. A decrease in transmittancevan significant compared to the case in which Ca was used. It isbelieved that the phenomenon was caused by Al which had highconductivity and high reflectance.

[0068] As has been thus described in detail, according to the presentinvention, the transmittance in the visible region is improved, sealingdefects are reduced, and the influence of oxygen, water, and the like inthe outside air can be avoided as much as possile. In addition, as atransmissive type organic EL device, a device can be realized which hasa long life, is driven at a low voltage, and has a high transmittance inthe visible light region.

What is claimed is:
 1. An organic electroluminescent device comprising an anode, a light-emitting layer composed of an organic material, and a cathode having a structure in which a first cathode composed of a material having a work function of 3.0 eV or less and a second cathode composed of a material having a work function higher than that of the first cathode are sequentially stacked from the light-emitting layer side are stacked on a substrate, the total thickness of the first and the second cathodes being 100 angstroms or less, and light is emitted to the outside via at least the cathode.
 2. An organic electroluminescent device according to claim 1, wherein the cathode side is sealed by a sealing layer composed of a light transmissive material.
 3. An organic electroluminescent device according to claim 1, wherein the first cathode comprises Ca.
 4. An organic electroluminescent device according to claim 1, wherein the thickness y (angstrom) of the first cathode is such that 50≦y≦80 holds.
 5. An organic electroluminescent device according to claim 1, wherein the thickness y (angstrom) of the first cathode is such that 55≦y≦65 holds.
 6. An organic electroluminescent device according to claim 1, wherein the second cathode comprises Al.
 7. An organic electroluminescent device according to claim 1, wherein the thickness z (angstrom) of the second cathode is such that 10≦z≦20 holds.
 8. An organic electroluminescent device according to one of claims 1 to 7, wherein the organic material forming the light-emitting layer is a polymeric material.
 9. A method for manufacturing an organic electroluminescent device, comprising: a step of forming an anode on a substrate; a step of forming a light-emitting layer composed of an organic material above the anode; and a step of forming a cathode above the light-emitting layer by laminating a first cathode composed of a material having a work function of 3.0 eV or less and a second cathode composed of a material having a work function higher than that of the first cathode from the light-emitting layer side so that the total thickness of the first and the second cathodes is 100 angstroms or less.
 10. A method for manufacturing an organic electroluminescent device according to claim 9, wherein the step of forming the anode further comprises a step of performing an oxygen or an air plasma treatment after an electrode film is formed, and a current x and a time t in the treatment are set such that 10 (mA)≦x≦15 (mA) and 5 (minute)≦t≦7 (minute) hold.
 11. A method for manufacturing an organic electroluminescent device according to claim 9, wherein the step of forming the anode further comprises a step of performing at oxygen or an air plasma treatment after an electrode film is formed, and a current x and a time t in the treatment are set such that 10 (mA)≦x≦12 (mA) and t=5 (minute) hold. 